Structure and method of preventing electrolytic corrosion for magnesium alloy member

ABSTRACT

A structure and a method of preventing electrolytic corrosion for a magnesium alloy member ( 20 ), the structure wherein a first coated layer ( 11 ) formed by electro deposition and a second coated layer ( 12 ) formed by distributing PTFE particles on the first coated layer ( 11 ) are covered on the surface of a tightening member ( 1 ) at least on a surface coming into contact with the magnesium alloy member ( 20 ), whereby, the electrolytic corrosion of the magnesium alloy member can be prevented at a low cost by insulating a tightening member such as a steel bolt and a washer from the magnesium alloy member, and an adhesiveness therebetween can be sufficiently assured.

TECHNICAL FIELD

[0001] The present invention relates to a technology for preventing theoccurrence of electrolytic corrosion in fastening parts in a fasteningstructure for a magnesium alloy member and a fastening member made of ametal other than that of the magnesium alloy member.

BACKGROUND ART

[0002] In the automobile industry, recently, there have been anincreasing demands for fuel economy as concerns about environmentalproblems mount. To meet such demands, the automobile industry isresearching ways to reduce the weight of car bodies, and is attemptingto increase the use of magnesium alloy in automotive parts because of itis lightest in weight among metals which may be practically used.Recently, in particular, it has been research for use in parts demandingvery high corrosion resistance, such as the outer housing and structuralparts.

[0003] However, since magnesium alloy is the most common practicalalloy, when fastened together with different metals such as iron andaluminum, electrolytic corrosion is likely to occur in the presence ofmoisture containing electrolytes. In particular, in the engine space andat the underside of an automobile, electrolytic corrosion is extremelypromoted by the action of electrolytes contained in rainwater, meltingsnow, salt, etc., and problems, that is, loosening, may occur in thefastened parts. Hitherto, as disclosed in Japanese Patent No. 2715758,aluminum washers were insulated by anodic oxidation, or bolts werecoated with resin as disclosed in Japanese Patent Publication No.S58-40045.

[0004] However, anodic oxidation of aluminum washers is very expensive.In the case of coating bolts with resin, contact adhesion of resincoating films on bolts and durability are insufficient, and the coatingfilm may peel off, resulting in electrolytic corrosion, and improvementin contact adhesion is required.

DISCLOSURE OF THE INVENTION

[0005] It is therefore an object of the invention to provide a structureand a method for resistance to electrolytic corrosion which can assuresufficient contact fastening of a magnesium alloy member securely and atlow cost while preventing electrolytic corrosion by insulating afastening member such as steel bolt or washer.

[0006] The structure which is resistant to electrolytic corrosion for anmagnesium alloy member of the invention is characterized by coating atleast the surface of a fastening member which is to contact with themagnesium alloy member with a first coating layer by electro deposition,and coating a second coating layer having polytetrafluoroethyleneparticles (PTFE particles) dispersed therein on the first coating layer.

[0007] The method of preventing electrolytic corrosion of a magnesiumalloy member of the invention is characterized by the steps of applyinga first coating layer by electro deposition on at least the surface of afastening member which is to contact the magnesium alloy member, and astep of applying a second coating layer having PTFE particles dispersedtherein on the first coating layer, thereby forming a crosslinkingstructure of the first coating layer and second coating layer.

[0008] According to the invention, the first coating layer formed byelectro deposition has outstanding contact adhesion on the fasteningmember and outstanding durability compared with conventional dipcoatings. Therefore, the first coating layer is difficult to peel offthe surface of the fastening member, and electrolytic corrosion isthereby prevented effectively. The second coating layer formed bydisposing PTFE particles is crosslinked to the first coating layer andis firmly adhered to the first coating layer. The second coating layerhas extremely low frictional resistance, and the contact adhesion anddurability thereof are extremely high. In addition, since the secondcoating layer is water repellent, the electrolytic corrosion preventiveeffects and weatherability of the coating are enhanced.

[0009] When the fastening member is a bolt, since the frictionalresistance is low, variation in friction in fastening is reduced.Accordingly, the fastening torque is stable when fastening the bolt,variations in the axial force of the bolt are suppressed, and a uniformaxial force is obtained. Hitherto, it was difficult to obtain a uniformaxial force in a completely degreased state, or in a state contaminatedwith coolant, rust preventive, or other oil or grease, or when surfaceconditions varied; however, since the second coating layer forming thesurface has low frictional resistance and is water repellent, a uniformaxial force is obtained, regardless of surface conditions.

[0010] The material for the first coating layer of the inventionincludes various resins such as cationic or anionic epoxy, acrylic,polybutadiene, and alkyd resins; cationic epoxy resins are preferablyused from the viewpoint of high corrosion resistance and contactadhesion. The thickness of the first coating layer should be 5 μm ormore in order to assure contact adhesion and durability; however, if thethickness exceeds 50 μm, uniform thickness may not be obtained, andimprovement of effect is not expected, and the electro depositionconsumes too much energy. Therefore, the thickness of the first coatinglayer is preferred to be 5 to 50 μm, or more preferably 20 to 50 μm.When forming the first coating layer on the fastening member, in thecase in which the fastening member is made of steel, it is preferred toform a base coat by forming a film of phosphate or black oxide. As thebase coat, a Zn or Cr plating may also be applied.

[0011] The second coating layer of the invention is formed by dispersingPTFE particles in a synthetic resin and an organic solvent such asalcohol or ketone in order to adhere more firmly to the first coatinglayer, and drying, and the concentration of PTFE particles in thesolvent is, for example, 1 to 30%. At this time, the content of thesynthetic resin is preferred to be 10 to 50% of the solid content of thePTFE. In order that the second coating layer may exhibit a desired lowfrictional coefficient, the molecular weight of PTFE particles ispreferred to be 1000 or less, and the particle size should be 1 μm orless. The thickness of the second coated layer is preferred to be 1 to10 μm in order to obtain durability and stability of frictional torque.These materials for the first coating layer and second coating layer areinexpensive, and therefore the invention is realized at low cost.

[0012]FIG. 1 is a sectional view showing the concept of the invention,in which a base coat 2 is applied on the surface of a fastening member 1such as a steel bolt, and a cationic epoxy resin is applied on thesurface of the base coat 2 by electro deposition, and a first coatinglayer 11 is formed. After drying the first coating layer 11, the firstcoating layer 11 is immersed for a specified time in a solvent in whichPTFE particles are dispersed, and the first coating layer 11 and secondcoating layer 12 are heated and cured. By curing, the PTFE particles arecrosslinked and held on the surface of the first coating layer 11, and acrosslinked structure is formed. The fastening member 1 is fastened whenthe second coating layer 12 contacts magnesium alloy member 20.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a sectional view showing schematically structure forpreventing electrolytic corrosion of the invention.

[0014]FIG. 2 is a diagram explaining the testing method by aring-on-disk method, in which (a) is a perspective view of a test piece,and (b) is a side view showing the device schematically.

[0015]FIG. 3 is a diagram showing results of contact adhesion of theExample and the Comparative Examples by the ring-on-disk method.

[0016]FIG. 4 is a diagram showing results of contact adhesion of thefirst coating layer in the Example by the ring-on-disk method.

[0017]FIG. 5 is a diagram showing results of contact adhesion of thesecond coating layer in the Example by the ring-on-disk method.

[0018]FIG. 6 is a diagram showing changes of axial force in the Exampleand in the prior art.

[0019]FIG. 7 is a diagram showing changes in axial force in an oiledstate and in a degreased state of the Example and the prior art.

[0020]FIG. 8 is a schematic side view of a testing device of theball-on-disk method.

[0021]FIG. 9 is a diagram showing results of testing changes offrictional coefficient of the Example by the ball-on-disk method.

BEST MODE FOR CARRYING OUT THE INVENTION

[0022] Actions and effects of the invention are described below withreference to an embodiment.

[0023] (1) Test by Ring-on-Disk Method

[0024] A. Adhesion Test of Surface Layer

[0025] Referring first to FIG. 2, the testing method by a ring-on-diskmethod is explained. Test pieces are disk 1 and ring 2 as shown in FIG.2(a), and as shown in FIG. 2(b), while rotating the disk 1 about theshaft by a drive source 10, the end face of the ring 2 is pressedagainst a surface thereof at a specified pressure, and changes offrictional torque on the basis of the driving torque for rotating thedisk 1 are measured.

[0026] According to the descriptions shown in Table 1, coating layerswere formed on the surface of steel disks 50 mm in diameter and 1 mmthick, and test pieces of the Example and Comparative Examples 1 to 4were obtained. While rotating the disks about the shaft at a speed of 20rpm, the end face of a magnesium alloy ring of Ra 0.13 to 0.20 μm, 20 mminner diameter and 25.6 mm outer diameter was pressed against thesurface, and while raising the pressing load at a rate of 100 kgf/min,changes in frictional torque (kgf-cm) on the basis of the driving torquefor rotating the disk were measured. Results of measurement are shown inFIG. 3. TABLE 1 Coating layer Coating Baking Coating layer CoatingBaking 1 method condition 2 method condition Example Cationic Electro150° C. PTFE disperse Dip 190° C. epoxy deposition  10 min solution  30min Comparative Cationic Electro 190° C. None — — Example 1 epoxydeposition  30 min Comparative Solvent type Dip 190° C. None — — Example2 epoxy  40 min Comparative Anionic Electro 190° C. None — — Example 3epoxy deposition  40 min Comparative Cationic Electro 190° C. PTFEdisperse Dip 190° C. Example 4 epoxy deposition  30 min solution  30 min

[0027] The strength resisting shear peeling is judged to be inferiorwhen the frictional torque is high as compared with the load of thering, and excellent when the frictional torque is low. As shown in FIG.3, in Comparative Example 1, the frictional torque rises in a range ofrelatively low load, and in Comparative Examples 2 and 3, the firstcoating film peels off at a lower load. In these Comparative Examples 1to 3 in which only the first coating layer is formed, the coating layerformed by immersing the solvent type epoxy resin (Comparative Example 2)is worst in contact adhesion, and the adhesion is increased in the orderof the coating layer formed by electro deposition of anionic epoxy(Comparative Example 3) and the coating layer formed by electrodeposition of cationic epoxy (Comparative Example 1). That is, thecationic epoxy is preferred as the resin and the electro deposition isrecommended as the forming method. In Comparative Example 4 coated withthe second coating layer after curing the first coated layer, thefrictional torque is higher than in the Example of the crosslinkedstructure of the first coating layer and second coating layer, and thecontact adhesion is inferior. In the Example, if the load is increased,the frictional torque is increase only very slightly, and the contactadhesion is excellent as compared with the Comparative Examples.

[0028] B. Adhesion Test of Each Coating Layer in the Example

[0029] In the Example, the first coating layer was prepared in fivethicknesses of 3 μm, 5 μm, 20 μm, 50 μm, and 70 μm, and in these firstcoating layers, the frictional torque was measured similarly by thering-on-disk method. Furthermore, the second coating layer to belaminated on the first coating layer was prepared in five thick of lessthan 1 μm, 1 μm, 3 μm, 10 μm, and 15 μm, and in these second coatinglayers, the frictional torque was measured similarly by the ring-on-diskmethod. Results of the first coating layers are shown in FIG. 4, and theresults for the second coating layers are given in FIG. 5. According toFIG. 4, in the thickness range of the first coating layer of 5 to 50 μm,there is no significant change in contact adhesion, and a favorableadhesion is assured. According to FIG. 5, in the thickness range of thesecond coating layer of 1 to 10 μm, uniformity of frictional torque wasconfirmed.

[0030] (2) Durability Test of Resin by Salt Spray Test

[0031] A base coat was applied on the surface of steel test pieces, andthe first coating layer was formed on the base coat by electrodeposition of resins, such as cationic or anionic epoxy resin, acrylicresin, polybutadiene resin, and alkyd resin, and salt water was sprayedon the coating layers for a specified time, and occurrence of rust wasinvestigated. The test method conforms to JIS K 5400. Results are shownin Table 2. In Table 2, results were evaluated as ⊚: no rust, ◯: smallspots of rust, and Δ: signs of rust, but no practical problems. TABLE 2Polybutadiene Alkyd Epoxy resin Acrylic resin resin resin Pencilhardness 3H 2H 2H H Salt spray ⊚ ◯ ◯ Δ resistance

[0032] According to Table 2, the coating layer of epoxy resin was nottorn by pencil of hardness 3H, and therefore, it was high in strength.The coating layers by acrylic resin and polybutadiene resin were strongenough by a pencil of hardness 2H, and the coating layer by alkyd resinhad no practical problem by a pencil of hardness H. Therefore, theseresins can be used as resins for the first coating layer, and inparticular, the cationic epoxy resin is most suitable.

[0033] (3) Water Repellence Test

[0034] Purified water was dropped on the surface of the Example andComparative Examples 1 and 2 with a drop diameter of 2 mm, and thecontact angle of the water drop on each coating layer was measured.Results are shown in Table 3. The larger the contact angle, the higheris the water repellence. TABLE 3 Example Comparative Example 1Comparative Example 2 120° 51° 101°

[0035] According to Table 3, the second coating layer of the Example issuperior in water repellence to the coating layer of the prior art. Ascompared with the first coating layer, the second coating layer of theExample is extremely improved in water repellence, and the effect of thesecond coating layer as a dispersed layer of PTFE particles wasobserved.

[0036] (4) Measurement of Axial Force

[0037] Plural samples were prepared by forming the coating layer byapplying the Example on M8 flanged bolts, and these were engaged andfastened with nut members, and the fastening torque and axial force weremeasured. Conventional samples which were galvanized were similarlytested. Results are shown in FIG. 6. According to FIG. 6, as comparedwith conventional galvanized samples, variations in axial force wereslight in the bolts of the Example, and therefore adequate torquecontrol was judged to be possible.

[0038] (5) Measurement of Axial Force (Comparison Between Oiled Stateand Degreased State)

[0039] Plural samples were prepared by forming the coating layer byapplying the Example on M8 flanged bolts, and these were tested in anoiled state and in a degreased state, and the fastening torque and axialforce were measured. Conventional samples which were galvanized weresimilarly tested. Results are shown in FIG. 7. According to FIG. 7, ascompared with conventional galvanized samples, the axial force of thebolt of the Example was not significantly different between the oiledstate and the degreased state, and the water repellence was excellent,and a uniform axial force could be obtained regardless of the surfacecondition.

[0040] (6) Test by Ball-on-Disk Method

[0041] As the second coating layer, dispersed layers of three types ofPTFE particles which were different in molecular weight and particlesize were used, and their frictional coefficients were measured by theball-on-disk method. In the ball-on-disk method, as shown in FIG. 8,while rotating a disk 3 of magnesium alloy about the shaft by a drivesource 20, a steel ball 30 with a diameter of 10 mm on which was formedthe second coating layer was pressed and rolled. The force of the ball30 pulled in the rotating direction was detected by a sensor, and thefrictional coefficient was measured on the basis of this force. In thiscase, the load of the ball 30 pressed to the disk 3 was 100 g, and thespeed of the disk 3 for rolling the ball 30 was 0.2 m/sec. The threetypes of PTFE particles had a molecular weight of 1000 or less with anaverage particle size of 1 μm or less, a molecular weight of 300,000 to400,000 with an average particle size of 1 μm or less, and a molecularweight of 300,000 to 400,000 with an average particle size of 4 μm.Results are shown in FIG. 9. According to FIG. 9, in the case of PTFEparticles with a molecular weight of 1000 or less with an averageparticle size of 1 μm or less, the frictional coefficient was smaller byfar compared with the other two types of PTFE particles, and thereforesuch PTFE particles were confirmed to be most suitable.

1. A structure for preventing electrolytic corrosion of a magnesiumalloy member, the structure preventing electrolytic corrosion of themagnesium alloy member in contact with a fastening member of adissimilar material, characterized by: a first coating layer formed byelectro deposition on at least the surface of the fastening membercontacting the magnesium alloy member, and a second coating layer havingpolytetrafluoroethylene particles dispersed on the first coating layer.2. The structure for preventing electrolytic corrosion of a magnesiumalloy member according to claim 1, wherein the material of said firstcoating layer is a cationic epoxy resin.
 3. The structure for preventingelectrolytic corrosion of a magnesium alloy member according to claim 1,wherein the thickness of said first coating layer is 5 to 50 μm.
 4. Thestructure for preventing electrolytic corrosion of a magnesium alloymember according to claim 1, wherein the thickness of said first coatinglayer is 20 to 50 μm.
 5. The structure for preventing electrolyticcorrosion of a magnesium alloy member according to claim 1, wherein thethickness of said second coating layer is 1 to 10 μm.
 6. A method forpreventing electrolytic corrosion of a magnesium alloy member, allowinga magnesium alloy member to contact a fastening member of a dissimilarmaterial, characterized by the steps of: applying a first coating layerby electro deposition on at least the surface of a fastening membercontacting the magnesium alloy member, and applying a second coatinglayer having polytetrafluoroethylene particles dispersed on the firstcoating layer, thereby forming a crosslinked structure of the firstcoating layer and second coating layer.
 7. The method for preventingelectrolytic corrosion of a magnesium alloy member according to claim 6,wherein the material of said first coating layer is a cationic epoxyresin.
 8. The method for preventing electrolytic corrosion of amagnesium alloy member according to claim 6, wherein the thickness ofsaid first coating layer is 5 to 50 μm.
 9. The method for preventingelectrolytic corrosion of a magnesium alloy member according to claim 6,wherein the thickness of said first coating layer is 20 to 50 μm. 10.The method for preventing electrolytic corrosion of a magnesium alloymember according to claim 6, wherein the thickness of said secondcoating layer is 1 to 10 μm.